Chemical milling of silicon carbide

ABSTRACT

The method comprises the steps of (1) placing molybdenum on a silicon carbide specimen, (2) alloying the molybdenum into the silicon carbide, (3) performing, if desired, ion implantation or other process steps, utilizing the molybdenum as a mask, (4) etching away the molybdenum alloy such as by sodium peroxide. The method permits the use of low temperature operations.

United States Patent inventor Howard L. Dunlap Granada Hills, Calil. 769,013 Oct. 21, 1968 Sept. 2 1, 1971 Hughes Aircraft Company Culver City, Calif.

Appl. No. Filed Patented Assignee CHEMICAL MILLING OF SILICON CARBIDE 13 Claims, 10 Drawing Figs.

0.8. Ci 148/1-5, 156/17, 156/13, 204/143 Int. Cl. 1101i 7/50 FieldoiSearch 156/17, 13; 14811.5; 204/143 Primary Examiner-Jacob H. Steinberg Altorneys-iames K. Haskell and Lewis B. Sternfels ABSTRACT: The method comprises the steps of (1) placing molybdenum on a silicon carbide specimen, (2) alloying the molybdenum into the silicon carbide, (3) performing, if

desired, ion implantation or other process steps, utilizing the molybdenum as a mask, (4) etching away the molybdenum alloy such as by sodium peroxide. The method permits the use of low temperature operations.

PATENTEI] s'sm 197i 501 Fig. 10.

Howard L. Dunlap,

INVENTOR.

ATTORNEY.

CHEMICAL MILLING OF SILICON CARBIDE The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457.

The present invention relates to a method for shaping silicon carbide crystals and, more particularly, to such a method efiected by the removal of molybdenum alloyed into the silicon carbide crystal from the crystal. Before removal, the molybdenum alloy may be used as a mask.

Silicon carbide is an extremely difficult material to precisely shape and work. Conventional methods include optical polishing, etching by means of chlorine and oxygen in conjunction with a silicon dioxide mask, and sandblasting. Optical polishing is a mechanical grinding process which requires the use of special jigs to hold the sample. This polishing is very time consuming and leaves an undesirable damaged region on the crystal surface. Since such crystals are small, it may, in addition, not be possible to handle such small devices by use of jigs. Optical polishing is further limited as to the smallness of a flat desired. Utilization of chlorine and oxygen in conjunction with the silicon dioxide mask requires relatively high tempera tures and is extremely time consuming; nevertheless, it may not necessarily produce the desired shaping and the high temperature may damage any work previously performed in the crystal. It is not possible to produce accurate shaping by means of sandblasting.

In addition, it has not been heretofore possible to perform preetching operations, such as doping, before utilizing conventional shaping methods, since such methods would affect the doped effect.

The present invention overcomes these and other problems by providing a low temperature method for forming thin areas on silicon carbide crystals. Briefly, the present invention provides for the alloying of molybdenum into the silicon carbide crystal in the pattern desired to be formed into the crystal or specimen. This step is accomplished by placing molybdenum on the silicon carbide specimen and by heating them to a temperature sufficient to melt the molybdenum and to combine the molybdenum and silicon carbide. Alternatively, the molybdenum may be sputtered or evaporated onto the silicon carbide. Thereafter, the molybdenum alloy is etched out of the silicon carbide by means of a suitable etchant, such as sodium peroxide melt. The silicon carbide is thus shaped as desired. If desired, before the molybdenum alloy is removed by etching, other operations maybe performed on the silicon carbide, utilizing molybdenum as a mask. Such other operations may include ion implantation and it is possible to use such preetching steps because the present is performed at a low temperature.

It is, therefore, an object of the present invention to prepare shaped specimens of silicon carbide by a low temperature process means.

Another object is the provision of a method for shaping silicon carbide by utilizing a molybdenum-silicon carbide alloy.

Other aims and objects, as well as a more complete understanding of the present invention, will appear from the following explanation of exemplary embodiments and the accompanying drawings thereof, in which:

FIGS. 1-41 depict the inventive method for a simple configuration of alloying, FIGS. 1 and 2 respectively depicting a side and top view of a ring of molybdenum on a silicon carbide crystal, FIG. 3 depicting the molybdenum alloyed into the silicon carbide, and FIG. 4 depicting the molybdenum having been removed by etching from the silicon carbide to form a shaped crystal; and

FIGS. 5-10 depict a more complicated etching design, FIGS. 5-8 showing, respectively, top, end, bottom, and side views before the alloying of molybdenum, FIG. 9 depicting the molybdenum after having been alloyed into the silicon carbide and the implantation of dopant ions into the silicon carbide surface, and FIG. 10 showing the silicon carbide after the molybdenum has been removed therefrom.

Referring to FIGS. 1-4, a crystal 20 of semiconductor quality silicon carbide, which has been chemically cleaned, is shown with a ring 22 of molybdenum placed on the crystal. The molybdenum may be in the form of a wire or, if desired, the molybdenum may be in a powder form. Such a powder is first mixed with water or other binder to make a paste which is then placed on the silicon carbide crystal in the desired pattern. The paste is then dried in atmospheric conditions or in a furnace so as to completely evaporate: the water or other binding material. The silicon carbide and molybdenum are then ready for placement into a furnace for alloying.

Such a furnace must produce a temperature which is sufficiently high to obtain an alloying between the molybdenum and the silicon carbide. The furnace is sealed after the silicon carbide with molybdenum thereon is placed therein and an atmosphere sufficient to exclude oxygen is utilized, such an atmosphere, for example, comprising; hydrogen, helium, or vacuum. The furnace is heated to a temperature wherein the molybdenum powder melts and alloys into the silicon carbide, as shown in FIG. 3, the alloy forming a small indentation 24 in the silicon carbide crystal. The time involved in the formation of the alloy depends on the degree and depth of alloying desired. The alloyed molybdenum and silicon carbide is then removed from the furnace and cooled at a rate sufficient to prevent crystal damage.

The silicon carbide and alloyed molybdenum are then placed in an etchant solution to remove the molybdenum. Preferably, the solution comprises a melt of sodium peroxide which etches both the molybdenum and Silicon carbide; however, since the molybdenum etches at a much faster rate than the silicon carbide, the molybdenum'is primarily etched away to leave an etched surface 26, as shown in FIG. 4. In addition, any rough spots in the silicon carbide are removed by the etching action of the sodium peroxide. Thereafter, the shaped silicon carbide is washed in water and in a neutralizing acid, such as hydrochloric acid. The shaped silicon carbide specimen is then in condition for further operations requiring the use of an etched surface 26.

The process shown in FIGS. 5-10 depicts a process similar to that shown in FIGS. 1-4. In this illustrated embodiment of the inventive process, a thin silicon carbide portion is supported by a large region of silicon carbide. Such a configuration is useful in field effect and other high field devices.

FIGS. 5-8 depict a specimen 30 of silicon carbide onto which three individual pieces of molybdenum 32, 34, and 36 are placed, the pieces 32 and 34 placed on the upper surface of the silicon carbide specimen and the piece 26 being placed on the bottom side, Such molybdenum pieces may be placed thereon such as, for example, by utilizing molybdenum powder mixed with a binder, as described above with respect to FIGS. 1-4. In a similar manner as described above, the silicon carbide specimen with molybdenum is placed in a furnace and heated to a temperature sufficient to provide an alloying of the molybdenum in the silicon carbide, as shown in FIG. 9 to provide a specimen 30 having alloyed molybdenum portions 42, 44, and 46 representing the areas in which the molybdenum pieces 32, 34, and 36 have been placed.

At this point it may be desired to perform other operations on specimen 32, such as ion implantation which is schematically depicted by arrow 48. After implantation has been effected, the molybdenum alloy portions 42, 44, and 46 are removed, preferably by sodium peroxide, to produce a silicon carbide specimen 50 having doped surface areas 52.

Although the preferred etchant comprises sodium peroxide, since this material acts speedily, other etchants may be utilized such as a solution of hydrogen fluoride as an electrolytic etch for removing P-type silicon carbide, sodium hydroxide at 900 C, sodium carbonate at approximately 900 C, a combination of potassium carbonate and sodium carbonate at approximately l000 C, and a combination of potassium carbonate and potassium nitrate at approximately 900 C.

EXAMPLE A silicon carbide crystal having a flat-bottomed pit was produced by utilizing the present invention. A commercially available, semiconductor quality, single crystal of silicon carbide was chemically cleaned by use of trichlorethylene; however, other degreasers, etching materials, or optical polishing materials will produce suitably clean silicon carbide. A molybdenum powder was then mixed with distilled and deionized water to from a paste. This paste was applied on the silicon carbide surface. The molybdenum paste was dried so that water was evaporated in order to prevent steam from later disturbing the powder.

The molybdenum on the silicon carbide was then placed in a furnace having capability of obtaining a temperature of 3,000 C. The furnace was sealed and a flow of hydrogen gas was initiated. The furnace was heated to approximately l,700 C, at which point the molybdenum powder melted and alloyed into the silicon carbide. The precise temperature at which the molybdenum alloyed into the silicon carbide was not obtained. The silicon carbide and molybdenum were left in the furnace to an extent sufficient to obtain the proper alloying.

Thereafter, the hydrogen gas flow was turned off and the silicon carbide with alloyed molybdenum was cooled at a rate which was sufficiently slow so as to prevent cracking by differential cooling rate. The silicon carbide with alloyed molybdenum was then removed from the furnace and dipped in molten sodium peroxide at approximately 500 C, which is the approximate temperature at which sodium peroxide melts. The silicon carbide specimen was left in the sodium peroxide melt for approximately 3 seconds at which time the molybdenum was etched away to produce a flat-bottomed pit in the silicon carbide. The silicon carbide crystal was then washed in water and hydrochloric acid to neutralize any remaining sodium peroxide.

The resulting silicon carbide specimen had a flat-bottomed pit therein with little irregularity where the molybdenum alloy had been located.

in a second example, molybdenum wire of approximately 1.5 mils diameter was bent into the shape or geometry of the desired pattern and placed onto the silicon carbide. The process as set forth above was then followed and the resulting silicon carbide was provided with a moatlike etched portion where the molybdenum wire had been placed.

In a third example, before the molybdenum alloy was removed by etching, the silicon wafer was doped by ion implantation methods and, after the ions had been implanted into the silicon carbide, the whole was annealed at approximately l,200 C, with the molybdenum mask in place. Thereafter, the mask was etched from the sample. Etching was effected at room temperature, first by aqua regia and then by a solution of hydrofluoric, nitric and acetic acids to prevent deleterious effect to the carrier concentration profile obtained by ion implantation. Although the invention has been described with reference to particular embodiments thereof, it should be realized that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A method for removing material at desired locations from a specimen of silicon carbide comprising the steps of:

alloying molybdenum into the silicon carbide specimen at the desired locations in a sealed furnace having a hydrogen atmosphere therein and at a temperature sufficiently high at approximately l,700 C, and for a time sufficiently long to cause the molybdenum to form an alloy with the silicon carbide specimen at the desired locations, and

reacting the silicon carbide specimen and alloyed locations in a sodium peroxide melt for a time sufiicient to etch the alloy from the silicon carbide specimen at the desired locations.

2. A method as in claim 1 for etching a desired geometrical configuration into the specimen at the desired locations further comprising the step of shaping the molybdenum in the geometrical configuration desired to be etched into the specimen.

3. A method as in claim 1 further including the prereacting steps of utilizing the alloy as a mask and doping the alloyed crystal by ion implantation process to provide a doped area of the specimen at locations other than the desired locations when said reacting step is performed.

4. A method for chemically milling at least a portion of a specimen of silicon carbide comprising the steps of:

utilizing the silicon carbide specimen alloyed with molybdenum at, at least, the portion thereof and removing the alloyed molybdenum portion from the specimen.

5. A method as in claim 4 further comprising the steps of utilizing the molybdenum alloy portion as a mask and doping the specimen before said removing step.

6. A method as in claim 5 wherein said doping step comprises the use of ion implantation techniques.

7. A method as in claim 4 wherein said removing step comprises the use of an etchant at a temperature sufficiently high and for a time sufficiently long to produce a chemical reaction between said etchant and the alloy.

8. A method as in claim 7 wherein said etchant consists of the group selected from sodium hydroxide, borax, sodium carbonate, potassium carbonate, sodium peroxide and combinations thereof.

9. A method as in claim 7 wherein said etchant consists of a sodium peroxide melt at a temperature of approximately 500 C.

10. A method as in claim 4 further comprising the step of preparing the silicon carbide specimen having the molybdenum alloy therein, said preparing step comprising the steps of placing molybdenum on a silicon carbide crystal and heating the molybdenum and the silicon carbide crystal sufficiently to alloy the molybdenum into the silicon carbide crystal.

11. A method as in claim 4 comprising the prealloying step of placing the molybdenum in the specimen.

12. A method as in claim 11 wherein said placing step comprises the steps of utilizing a molybdenum paste and drying the paste on the specimen.

13. A method as in claim 11 wherein the portion is provided with a specified geometrical configuration further comprising the step of shaping the molybdenum in the specified geometrical configuration desired to be etched into the specimen. 

2. A method as in claim 1 for etching a desired geometrical configuration into the specimen at the desired locations further comprising the step of shaping the molybdenum in the geometrical configuration desired to be etched into the specimen.
 3. A method as in claim 1 further including the prereacting steps of utilizing the alloy as a mask and doping the alloyed crystal by ion implantation process to provide a doped area of the specimen at locations other than the desired locations when said reacting step is performed.
 4. A method for chemically milling at least a portion of a specimen of silicon carbide comprising the steps of: utilizing the silicon carbide specimen alloyed with molybdenum at, at least, the portion thereof and removing the alloyed molybdenum portion from the specimen.
 5. A method as in claim 4 further comprising the steps of utilizing the molybdenum alloy portion as a mask and doping the specimen before said removing step.
 6. A method as in claim 5 wherein said doping step comprises the use of ion implantation techniques.
 7. A method as in claim 4 wherein said removing step comprises the use of an etchant at a temperature sufficiently high and for a time sufficiently long to produce a chemical reaction between said etchant and the alloy.
 8. A method as in claim 7 wherein said etchant consists of the group selected from sodium hydroxide, borax, sodium carbonate, potassium carbonate, sodium peroxide and combinations thereof.
 9. A method as in claim 7 wherein said etchant consists of a sodium peroxide melt at a temperature of approximately 500* C.
 10. A method as in claim 4 further comprising the step of preparing the silicon carbide specimen having the molybdenum alloy therein, said preparing step comprising the steps of placing molybdenum on a silicon carbide crystal and heating the molybdenum and the silicon carbide crystal sufficiently to alloy the molybdenum into the silicon carbide crystal.
 11. A method as in claim 4 comprising the prealloying step of placing the molybdenum in the specimen.
 12. A method as in claim 11 wherein said placing step comprises the steps of utilizing a molybdenum paste and drying the paste on the specimen.
 13. A method as in claim 11 wherein the portion is provided with a specified geometrical configuration further comprising the step of shaping the molybdenum in the specified geometrical configuration desired to be etched into the specimen. 